1. Field of the Invention
The present invention relates to a method for forming a protective film and, then, performing marking on the protective film, and a sheet for both protective film forming and dicing which is favorably used in the method.
2. Description of the Prior Art
Recently, production of semiconductor devices is made through a so-called face down mounting process. In the face down process, chips are electrically connected with a substrate through a convex portion called as a bump formed on a circuit surface of the chip to ensure its conductivity to the substrate.
Semiconductor devices are generally produced though the following steps:
(1) forming a circuit on a surface of a wafer by etching or the like and providing a bump on a given position of the circuit surface;
(2) grinding a back surface of the wafer to have a given thickness;
(3) fixing the back surface of the wafer onto a dicing sheet which is tensely supported by a ring frame, and dicing the wafer to separate each circuit by the use of a dicing saw to obtain semiconductor chips; and
(4) picking up the semiconductor chips to mount them face down on a given substrate and sealing each chip in a resin or coating the back surface of the chip with a resin for chip protection according to necessity, thereby obtaining a semiconductor device.
Such resin sealing is performed by dripping a resin in a proper amount on the chip (potting method) or using a mold (molding method), both followed by curing. The potting method has a drawback of difficulty in dripping the proper amount of the resin. The molding method involves washing of the mold, which will require additional costs for equipment and operation thereof. Such resin coating may cause uneven quality because of difficulty in spreading a proper amount of resin uniformly on the chip. Therefore, the technique which is capable of forming a highly uniform protective film on the back surface of the chip by a simplified operation has been desired.
In grinding the back surface of the wafer in step (2), minute streaky scratches are formed on the back surface of the chip owing to the use of a grinding machine. The minute scratches may cause cracks during the dicing in the step (3) or after the device is packaged. As such, it has been conventionally required in some cases to perform chemical etching after the mechanical grinding to eliminate the minute scratches. The chemical etching, as a matter of course, raises problems related to the cost increase for its equipment and operation. Therefore, the technique for preventing adverse effects resulting from minute scratches has been desired, even if minute scratches are left on the back surface of the wafer as a result of mechanical grinding.
As a technique for responding to such requirements, JP-A No. 2002-280329 discloses “a sheet for protective film forming for chips having a release sheet and a protective film forming layer which is formed on a release surface of the release sheet and contains a thermosetting component and/or an energy ray-curable component and a binder polymer component”. In JP-A No. 2004-214288, it has been described that, further in the invention according to JP-A No. 2002-280329, in order to enhance an adhesion property between a protective film formed by curing an protective film forming layer and a wafer (chip) which is an adherend, a curable adhesion layer is provided on the protective film forming layer.
In a process using the sheet for the protective film forming for chips, the protective film forming layer can be formed on the wafer such that the sheet for forming the protective film for chips is adhered on the wafer and, then, a release sheet is removed. Next, the protective film forming layer on the wafer becomes a protective film by curing with heating or the like and, then, marking of, for example, a product number is performed on the protective film. Thereafter, the wafer having the protective film is fixed on a dicing sheet and, then, dicing and pick-up are performed. As for a marking method, ordinarily, a laser marking method which inscribes a surface of the protective film by irradiating laser light is used.
In the above-described process, there has been a case in which, when the sheet for the protective film forming layer for chips is adhered to the wafer, tension applied to the sheet remains after it is adhered to cause the wafer to have a warpage. Another case in which, when the protective film is cured, the protective film is shrunk to cause the wafer to have a warpage. Still another case in which the wafer itself to be used originally has a warpage.
In the above-described process, when the marking is performed, laser light can not be focused on the wafer having such warpage as described above and, accordingly, the marking having a high accuracy has not been performed.
In light of the above prior art, the present invention has been achieved and has an object to provide, in a process in which marking is performed on a protective film formed on a work such as a wafer, a marking method in which the marking is performed on the protective film with a high accuracy by suppressing a warpage and, also, a sheet for both protective film forming and dicing which is advantageously used in the method.